Fluorescent panel lamps with white emitting phosphor coated on envelope backplate and red emitting phosphor coated on envelope faceplate



NOV. 5, 1968 w c MARTYNY ET AL 3,409,792

FLUORESCENT PANEL LAMPS WITH WHITE EMITTING PHOSPHOR COATED ON ENVELOPE BACKPLATE AND RED EMITTING PHOSPHOR COATED ON ENVELOPE FACEPLATE Filed Nov. 15, 1965 2 Sheets-Sheet 1 wimam c. Mav tgng Pevc d.U derwood b9 Their Aft tovneg NOV. 5, 1968 q; MARTYNY ET AL 3,409,792

FLUORESCENT PANEL LAMPS WITH WHITE EMITTING PHOSPHOR COATED ON ENVELOPE BACKPLATE AND RED EMITTING PHOSPHOR COATED ON ENVELOPE FACEPLATE Filed NOV. 15, 1965 2 Sheets-Sheet 2 Fig 2.

6 I s I I a fir \wli I i ITWVGTWTCDTSI WiLLiam C. Mart9n9 Pevc J.Uncl vwood by M Their A t tznvney United States Patent 3,409,792 FLUORESCENT PANEL LAMPS WITH WHITE EMITTING PHOSPHOR COATED ON ENVE- LOPE BACKPLATE AND RED EMITTING PHOSPHOR COATED ON ENVELOPE FACE- PLATE William C. Martyny, Lyndhurst, and Percy J. Underwood,

Mentor-on-the-Lake, Ohio, assignors to General Electric Company, a corporation of New York Filed Nov. 15, 1965, Ser. No. 507,892 3 Claims. (Cl. 313-109) ABSTRACT OF THE DISCLOSURE To achieve a desired color rendition in a panel fluorescent lamp having appreciably more surface area in the backplate than in the faceplate, it is preferable to coat different phosphors ,on the backplate and the faceplate and allow the light to blend, rather than to mix the phosphors and coat both plates with the mixture. The phosphor which is more efiicient or less expensive than the other is coated more thickly on the backplate which operates in part as a reflector, and the other is coated more thinly on the faceplate which operates primarily to transmit light. This arrangement permits a gain in efliciency or a reduction in cost.

This invention relates to directional fluorescent lamps which emit light preferentially in certain directions, and more particularly to phosphor combinations used therein for achieving a desired color rendition.

Directional fluorescent lamps are available in tubular form and are generally known as reflector lamps. In some a reflective coating, for instance titanium dioxide, is used on part of the circumference and the phosphor coating is then applied internally over the entire circumference. In others no reflective coating as such is used but the phosphor coating is thicker over part of the circumference so that more light is transmitted through the thinner portion of the coating.

The panel fluorescent lamp is also a directional lamp. In this lamp the discharge path is a labyrinthine channelway formed between two vitreous components, one known as the faceplate and the other as the backplate, sealed together along their margins to make the envelope, the plates being coated internally with phosphor. A relatively long discharge occurs in a small area and this is an advantage from the point of view of compactness of source.

In the applications visualized for a panel lamp, it is generally intended that only one face of the lamp be exposed to view. For this reason, an attractive appearance is desired for the faceplate and it is made relatively flat. In a preferred panel fluorescent lamp construction offered commercially by applicants assignee, the faceplate is provided with a pattern of shallow embossments of square outline giving somewhat the appearance of a quilt or checkerboard and this improves both the appearance and the strength or rigidity of the panel. Since the faceplate is relatively flat, the backplate must be molded deeply to define the labyrinthine discharge channelway 'by means of parallel grooves extending side by side and joined together at alternate ends. The surface area of the discharge space bounded by the backplate is much greater than that bounded by the faceplate, and if the fluorescent phosphor coating is applied to both plates with equal thickness, only about one-third of the total light is projected from the face of the lamp while about two-thirds is projected from the back. This is undesirable because it means ineflicient utilization or loss of light when the lamp is used in a recessed fixture. The situation has been corrected by coating the backplate with a thick layer-of phosphor and the faceplate with a thin layer. For instance by coating the backplate to a' thickness providing a reflectance of 66% while the faceplate is coated to areflectance of 50% only, .the proportion oftotal light projected from the faceplate increases to about 60%. while that projected from the backplate drops to 40%.

Many conventional tubular fluorescent lamps use mixtures or blends of phosphors to obtainvarious colors or spectral renditions. This has been the case particularly with the deluxe lamps which are essentialy white but have enhanced red emission. For instance the warm-white deluxe lamp in the conventional tubular construction uses a mixture of green-emitting strontium halophosphate and red-emitting strontium magnesium orthophosphate phosphors. I H

The object of this invention is to provide a more .eflicient and more effective and economical utilization of phosphors in directional fluorescent lamps, particularly in panel fluorescent lamps, where more than one kind of phosphor is used in order to achieve a desired color rendition or spectral output.

In accordance with the invention, a desired color rendition is achieved by coating different phosphors on different portions of the directional fluorescent lamp and allowing the light to blend, in preference to the former practice of mixing the phosphors. One phosphor component is coated on the portion of the lamp intended as a reflector and the other phosphor component on the portion intended primarily to transmit light. Where one phosphor component is more eflicient or less expensive than the other, the invention makes possible an increase in efliciency or a cost saving. In such case, the more eflicient or less expensive phosphor is used in a thick layer of higher reflectance on the reflector portion, while the less efiicient and more expensive phosphor is used in a thin layer of lower reflectance on the light transmitting portion.

By Way of example, in a panel fluorescent lamp the more eflicient or less expensive phosphor is provided as a relatively thick layer on the backplate which operates in part as a reflector, and the less eflicient or more expensive phosphor is applied as a thin layer on the faceplate. Thus in a panel deluxe fluorescent lamp of the Warm-white type, a relatively thick layer of white calcium halophosphate is applied to the backplate and a relatively thin layer of red-emitting strontium magnesium orthophosphate is applied to the faceplate. Surprisingly the light transmitted through the faceplate appears white rather than red. The explanation resides in the fact that the transmitted light is a mixture of light generated in the faceplate phosphor and both generated and reflected light from the backplate phosphor. Less of the relatively expensive red phosphor is used and the combination provides the desired color rendition at a higher efiiciency and at lower cost than when a phosphor blend or mixture is used.

For other features and for further objects and advantages of the invention, attention is now directed to the following detailed description of preferred embodiments to be taken in conjunction with the accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawing:

FIG. 1 is a plan view of the faceplate of a square panel fluorescent lamp embodying the invention.

FIG. 2 is a plan view of the backplate of the lamp.

FIG. 3 is a side view partly sectioned through the same lamp.

Referring to the drawing, the illustrated panel fluorescent lamp corresponds to the commercially available lamp designated FPIZS, 12" square and rated at 80 watts nominal for normal operation. The envelope 1 is made up of a pair of molded glass plates, a faceplate 2 and a backplate 3 sealed together along their margins. The backplate is molded into six parallel grooved sections or channels 4 extending side by side and joined together in alternating fashion by short sections 5 where the lengthwise partitions 6 end, thereby forming a continuous labyrinthine channelway. The faceplate is generally planar but configurated into a pattern of shallow square outwardly convex embossments 7 giving it somewhat the appearance of a grid or checkerboard. The quilting of the faceplate improves the appearance and increases the strength of the assembly.

The edges of the faceplate and backplate are hermetically heat-sealed together into a marginal ledge 3 which extends around the four sides of the lamp. Along the internal junctures between the partition 6 in the backplate and the flat areas or lands 9 between embossments in the faceplate, the glass surfaces are merely pressed together into close conformance. This is sufiicient to prevent the electric discharge from leaking through and compels the discharge to follow the channel through from end to end. Discharge supporting electrodes 10, 10 are provided at opposite ends of the channelway comprising a tungsten filament coated with activating material; they are supported on inleads 11 sealed through the marginal ledge and connected to the terminals of bases 12, 12. The bases may be of the kind described and claimed in copending application Ser. No. 106,827, filed May 1, 1961, of Albert P. Pate et al., entitled Panel Lamp Base, and assigned to the same assignee as the present invention, now Patent 3,253,176. The bases are attached to the marginal ledge 8 by means of clips 13 and silicone rubber cement where the lower side of the base body contacts the glass of the lamp. In use, the base terminals are engaged by a telescoping bar-type connector (not shown) which is placed to overlie the marginal ledge between the bases.

The lamp is provided with the usual ionizable filling consisting of mercury and an inert starting gas such as argon at a pressure of a few millimeters through vitreous exhaust tubes at the ends of the channelway which are tipped off as indicated at 14. An electric discharge through the lamp produces ultraviolet radiation which excites the phosphor coatings 15, 16 on the inside of the envelope. The proportion of the discharge bounded by the backplate is considerably greater than that bounded by the faceplate, more light is generated by the phosphor layer on the backplate and, with phosphor layers of equal thickness, more light is transmitted through the backplate than through the faceplate. In order to cause the lamp to transmit a greater proportion of its light downwardly through the faceplate than upwardly through the backplate, phosphor coating 15 applied to the faceplate is made thinner and has a lower reflectance than coating 16 applied to the backplate. Desirably the phosphor coating covers the entire faceplate including the flat areas or lands 9 between embossments in order to provide a uniformly white appearance to the faceplate.

In accordance with our invention, in a panel fluorescent lamp wherein the desired color rendition requires a blend of light from several phosphors, one of which is more efiicient or less expensive than the other, the more eflicient or less expensive phosphor is applied in a relatively thick coat 16 to the backplate, and the less eflicient or more expensive phosphor is applied in a thin coat 15 to the faceplate. In a preferred embodiment known as a panel deluxe lamp (warm-white type), the backplate is coated with white calcium halophosphate phosphor to a reflectance of 66% and the faceplate is coated with red-emitting strontium magnesium orthophosphate tin-activated phosphor to a reflectance of 50%. The light from the faceplate of this lamp falls within the warm white deluxe ICI color oval. Surprisingly the total light output from the lamp is 37.5% greater than from a similar panel lamp utilizing on both backplate and faceplate the conventional 4 warm-white deluxe phosphor mix, the total lumen output being 4100 for an input to the lamp of 77 watts.

A partial explanation for the high lumen values attained in the panel lamp ofthe invention is as follows. The light output of similar panel, lamps coated. entirely, withwhite calcium halophosphate only is -4 750:lumen s, andpoated entirely with red-emitting strontium magnesium orthophosphate only, is 2850 lumens. In the lamp accordingto the inventionythe backplate iscoated withwhite phosphor which amounts to about 65 of'the total phosphor area in the lamp and the'light generated in this'area is about 3100 lumens. The faceplate coated withjred-emitting phosphor makes up the remaining 35% of the phosphor area and the light contributionfrom this area is approximately 1000 lumens. Adding both contributions gives 4100 lumens and this is the light output which has'been measured with thesubjectlamp.

By comparison, in the conventional warm-white deluxe lamp utilizing a phosphor mixture, the same'colorpoint is achievedby mixing twoparts ofredremitting strontium magnesium orthophosphate and one part of green-emitting strontium halophosphate. As previously noted, the. light output from the red-emitting phosphor under equivalent conditions is 2850. lumens; the light output from the green-emitting phosphor under equivalent conditions is 4100 lumens. Assuming proportional light output from two parts of red and one part of. green, the result of addition is 3330 lumens for a panel lamp coated with such a mixture and this is in therange of the measured light output for such a lamp.

. The foregoing suggests that the large gain in lumen output in the lamp accordingto the invention is due'to a large extent to the lower percentage of red-emitting phosphor required. Surprisingly, the panel deluxe lamp according to the invention achieves the same color point as a conventionally made lamp but requires much less red phosphor, only about half as much-There are probably two reasons why the red phosphor is more effectively used in the lamp according to the invention. First by using a white phosphor on the backplate a good reflector is provided in respect of red light from the faceplate. Secondly, the light contribution of the thin layerof red-emitting phosphor on the faceplate is more effectively utilized and measurements indicate that of the red light generated in the lamp is transmitted through the faceplate. Therefore, the more eifective'utilization of the red light generated by the faceplate phosphor permits a reduction in the total amount of red-emitting phosphor used, and since the red-emitting phosphor is less efficient and more expensive, the final result is an increase in efficiency and a reduction in cost.

In another embodiment of the invention known as the panel deluxe cool white lamp, the backplate is coated with a phosphor mixture consisting of 82% by weight cool white calcium halophosphate, and 18% by weight blue calcium halophosphate. The faceplate is coated with a thin layer of red-emitting strontium magnesium orthophosphate tin-activatedphosphor. The light output from this lamp is approximately 30% greater than that of a conventionally made panel lamp using the standard cool white deluxe phosphor mixture or blend applied to both faceplate and backplate. 1

In yet another embodiment of the invention, a panel fluorescent lamp is designed as a plant growth or dis play lamp. This lamp uses magnesium fluorogermanate phosphor activated by manganese whose'output is concentrated in the red. The backplate of the lamp is coated with a phosphor mixture consisting of 55% by weight blue-emitting calcium tungstate and 45% by weight redemitting strontium magnesium orthophosphate. Thefaceplate is coated with a phosphor mixture consisting of 50% by weight of the foregoing mixture and 50% by weight of red-emitting magnesium fluoroge rmanate. The total lumen output and the so-called red lumens from this lamp are appreciably greater than can be achieved by mixing all the phosphor components together and applying the mixture to the envelope walls in a uniform manner.

In yet another embodiment of the invention a panel lamp has a radiant emission designed to attract insects in cotton and tobacco fields. This lamp is produced by coating the backplate with a longwave ultraviolet radiation producing phosphor such as barium magnesium strontium silicate activated with lead, and the faceplate with a blue-emitting strontium halophosphate. The total radiant output from this lamp again is greater than is achieved when both phosphor components are mixed together and the lamp is merely coated internally with the mixture.

The invention thus provides a new technique for increasing the light output and achieving the desired color rendition or spectral output from directional fluorescent lamps at no increase in cost and in many cases with a reduction in cost by reducing the amount of expensive phosphor component required.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A fluorescent panel lamp comprising a vitreous envelope formed by a backplate and a faceplate sealed together along their margins, said envelope containing an ionizable medium and having electrodes sealed therein for supporting a discharge which produces ultraviolet radiation, said backplate having a grooved channelway extending therein forming a continuous labyrinthine channel, said faceplate being relatively fiat, the proportion of the discharge channel area bounded by the backplate being about 65% and by the faceplate about Said backplate being coated primarily with a relatively effi cient white-emitting phosphor to a reflectance of about 66%, and said faceplate being coated with a red-emitting phosphor to a reflectance of about 2. A lamp as defined in claim 1 wherein the phosphor coated on the backplate is white calcium halophosphate and the phosphor coated on the faceplate is red strontium magnesium orthophosphate tin-activated.

3. A lamp as defined in claim 1 wherein the phosphor coated on the backplate is a mixture of about 82% by weight cool white calcium halophosphate and 18% by Weight blue calcium halophosphate, and the phosphor coated on the faceplate is red strontium magnesium orthophosphate tin-activated.

References Cited UNITED STATES PATENTS 2,135,732 11/1938 Randall et a1. 313109 2,346,522 4/1944 Gessel 313-109 2,445,692 7/1948 Porter et al 313109 3,115,309 12/1963 Spencer et al 313109 X 3,247,415 4/1966 Martyny 313-109 3,287,586 11/1966 Bickford 313-109 JAMES W. LAWRENCE, Primary Examiner.

P. C. DEMEO, Assistant Examiner. 

